parenting object to an object deformed by armature
You can select the teeth, then the body, then ctrlJ to join. But at this point, the teeth vertices won't be part of any vertex group, therefore no bone will be able to move them. So you also need to assign these vertices to the vertices group that they are supposed to be part of, i.e. the group that has the same name as the bone that controls it: Select the teeth vertices, go in the Properties panel > Data > Vertex Groups, select the right group and press the Assign button.

You don't need to join the 2 objects if you don't want to, in that case parent the teeth to the armature With Empty Group, then in Edit mode select the whole teeth mesh and assign it to the vertices group it is supposed to be part of.